@mny23  Nintex for SharePoint On-prem and Nintex for o365 did use the SharePoint workflow engine. Nintex Workflow cloud has it's own engine, you can find the product details and release history in the help(https://help.nintex.com/en-US/nwc/Content/Home.htm).

Thanks! @leighburke 

I could not find anything in the documentation regarding the technology that is used and also, when did they actually start to develop this...as far as I can remember, a few years ago, they did not have the cloud workflow as an independent engine...

@mny23 If you are looking for more in-depth technical information you would need to reach out to your account manager, if you are unsure who your account manager is you can ask support and they should be able to direct you. In terms of age of the product if you go into the release notes you will see it goes back to 2016(https://help.nintex.com/en-US/platform/ReleaseNotes/NintexWorkflowCloud.htm).

Thank you for your time! Keep in touch! Mihai